Regarding Harper, we see the same thing every single year. Some undrafted rookie receiver looks like a world beater in the second half of preseason games. It was Brian Tyms last year. Quentin Sims, Zach Sudfeld, Kenbrell Tompkins, Buddy Farnham, Ray Ventrone, Bam Childress. Some of those guys made the roster and contributed, even started, but none proved to be long for this world. This isn't anything new. Maybe Harper will be different, but I doubt it.
As for Gibson, he's certainly an NFL caliber receiver and a known quantity. He's just not a reliable player because of his terrible dropsies.